1. Field of the Invention
This invention relates to a current limiting ultra-fast contact breaker which can be used at medium voltage and although more particularly suited to direct current electrical traction in rolling stock or in fixed equipment, can also be used in alternating current applications.
2. Description of Related Art
It is well known that DC traction networks like those in industry are becoming more and more complex and powerful. The design of cut-off devices must evolve in order to be able to cut off increasingly larger currents and to reduce the maintenance costs. A cut-off device of the new generation must be fast in order to limit the current and reduce the mechanical and thermal requirements of the entire installation as well as the wear of its contacts and of its blowing box. At present, cut-off devices in traction networks include ultra-fast mechanisms for opening the contacts and a blowing box in which the arc created is confined and cooled. These devices give rise to significant costs due to maintenance operations and the replacement of worn parts.
In the European Patent Application No. 85 870 134.5, there is a description of an ultra-fast mechanism with electromagnetic holding wherein a same component serves as both repulsion disc and moving contact bridge, with a semiconductor-controlled oscillating circuit whose coil is used as a repulsion coil in the cut-off mechanism. The described assistance circuit, connected to the terminals of the mechanism, comprises a capacitor, a coil (repulsion coil) and a thyristor, connected in series together with a diode connected in anti-parallel with the components in series. The cut-off device fitted with the assistance circuit of this type is, however, only suitable for cutting off currents passing through the said device in the given direction. FIG. 6 of the Application 85.870 134.5 shows a similar assistance circuit intended for a bidirectional cut-off device which enables the cutting-off of current in both directions. It appears, however, that the efficiency of the cutting-off is substantially better in one direction of current flow than in the other. This is due to an asymmetry in the circuit from which it results that the second ogive of current produced by the capacitor, weaker than the first since it is already partially damped, must cut off a short-circuit current which has had an extended time in which to develop.